The goal of this research project is to characterize the organization of projections from the raphe nuclei to the vestibular system. Although fibers originating from raphe nuclei are known to innervate the vestibular nuclei, little is known about the specific organization of the raphe-vestibular pathway. The first part of this project will use anterograde tracing to examine the topography of raphe projections to the vestibular nuclei. However, because raphe projections arise from both serotonergic and non-serotonergic neurons, we have devised a novel combination of experimental techniques that will allow the termination patterns of the serotonergic and non-serotonergic components to be analyzed independently. The present research is also concerned with the organization of raphe projections to an ascending network that contributes to vestibular interactions with autonomic pathways. We will test the hypothesis that serotonergic and non-serotonergic raphe neurons are sources of collateralized axon projections to components of this ascending network - specifically the vestibular, lateral parabrachial, central amygdaloid, and hypothalamic paraventricular nuclei. We will use multiple retrograde tracer methods to identify these collateralized projections.